1. Field of the Invention
The present invention relates to an intraocular lens, and particularly to a self-centering, posterior chamber lens adapted for mounting in the capsule following extracapsular cataract extraction.
2. Description of the Prior Art
In the human eye, the lens is situated behind the pupil and iris, and functions to focus light entrant through the cornea and pupil onto the retina at the rear of the eye. The lens is a biconvex, highly transparent structure made of slender, curved rod-shaped ectodermal cells in concentric lamellae surrounded by a thin capsule. The lens capsule is supported at its periphery by suspensory ligaments, called zonules, that are continuous with the ciliary muscle. Contraction of this muscle relaxes the zonules, allowing the lens to become more spherical, thereby altering its focal length.
A cataract condition results when the material within the lens capsule becomes clouded, thereby obstructing the passage of light. To correct this condition, two forms of surgery are used. In intracapsular cataract extraction, the entire lens is removed intact. To accomplish this, the surgeon severs the zonules or suspensory ligaments about the entire periphery of the capsule, and removes the entire lens with the capsule and its content material intact.
In extracapsular cataract extraction, an incision is made through the front wall (the "anterior capsule") of the lens, and the clouded cellular material within the capsule is removed through this opening. Various scraping, suction or phacoemulsification techniques are used to accomplish such extraction. The transparent rear capsule wall (the "posterior capsule") remains in place in the eye. Also remaining in place are the zonules, and peripheral portions of the anterior capsule (the "anterior capsule flaps").
Both intracapsular and extracapsular extraction eliminate the light blockage due to the cataract. However, the light now entrant through the cornea and pupil is totally unfocused since there is no longer a lens in the eye. Appropriate focusing can be achived by a lens (i.e., a contact lens) exterior to the eye. This approach, though generally satisfactory, has the disadvantage that when the external lens is removed (i.e., when the contact lens is "taken out"), the patient effectively has no sight. A preferred alternative is to implant an artificial lens directly within the eye. One objective of the present invention is to provide such an intraocular lens.
Although at present more intracapsular lens removals are performed than extracapsular extractions, there are certain undesirable complications which may result from intracapsular surgery. The first involves "vitreous loss." The entire region of the eye behind the lens normally is filled with a jelly-like material called the vitreous humor. When the lens is removed intact, the vitreous humor comes up through the pupil and may escape from the eye through the incision that was made to accomplish the intracapsular extraction. Adverse side effects can occur.
Another complication of intracapsular surgery is called cystoid macula edema (CME). This is an edema or swelling of the macula of the retina. This may be due to certain enzymes which are released from the iris and migrate through the vitreous humor back to the macula, causing swelling. This is a serious complication. The incidence of both vitreous loss and CME is substantially reduced in the case of extracapsular extraction, since the posterior capsule remains in place and prevents the vitreous humor from reaching the anterior chamber. Thus from the viewpoint of reducing post-surgical complications, extracapsular extraction is preferred, and it is a further object of the present invention to provide an intraocular lens the use of which is particularly advantageous with extracapsular extractions.
Various forms of intraocular lenses are known. Generally these fall into two major classes, the anterior chamber lenses which are situated forward of, or mounted to the iris, and posterior chamber lenses which are situated behind the iris and may be mounted within the cleft or fornix of the capsule which remains in place after extracapsular surgery. The present invention is of the latter type.
Historically, the earliest posterior chamber lens implants were performed by Harold Ridley in the early 1950's. The Ridley biconvex lens was about the same shape, but had approximately 1 mm smaller diameter than the normal human lens. Its weight in air was 112 mg, an extremely heavy weight for an object to be implanted in the eye. The weight and relatively large diameter caused the Ridley lens to exert undue pressure on the ciliary body, the annular structure on the inner surface of the eye surrounding the lens and including the ciliary muscle and the ciliary process to which the zonules are connected. Other adverse side effects occurred. Glaucoma was noted. In come instances, the lens became loose and fell into the back of the eye. Many cases of downward decentration were noted, wherein the lens shifted downwardly so that its axis was no longer centered with respect to the pupil. For all of these reasons, the Ridley lens soon was abandoned.
A related lens designed by Strampelli for use in the anterior chamber also was tried in the early 1950's. This lens seated in the "angle" of the eye, where the cornea and iris are joined. Often, the use of such lens caused destruction of the endothelium, a very thin layer of live cells on the interior of the cornea. This is a very serious complication, and use of this form of angle-fixated anterior chamber lens soon was stopped.
Next came a series of anterior chamber lenses which were mounted to the iris. Amongst the earliest is the so-called Copeland or Epstein iris plane lens. This is a one-piece, cross-shaped structure having a generally plano-convex central lens and four planar blades projecting respectively from the top, bottom and sides of the lens. The lens is mounted in the pupil by inserting two diagonally opposite blades (usually the ones at top and bottom) behind the iris, and allowing the two remaining blades to seat against the front of the iris. Thus the structure actually is fixed directly to the iris. This eliminates the centration problem, since the lens itself is situated in the pupil, and also eliminates the problems associated with lenses that seat against the ciliary sulcus or against the angle. However, dilation of the pupil is difficult with this lens.
Another iris fixated lens was developed by C. D. Binkhorst, and is known as the "iris clip lens" or the Binkhorst four-loop lens. This lens includes a pair of anterior loops projecting from the top and bottom of the lens which seat against the front of the iris. A second pair of loops extend from the rear of the lens and are formed in an L-shape when viewed from the side. These posterior loops clip behind the iris. The Binkhorst iris clip lens is relatively easy to implant, and has the advantage of being very light. Typically, it weighs about 3 mg in aqueous. However, it has the disadvantage that when the pupil is dilated, the lens may fall out of place. In that instance, it could float forward against the cornea, destroying the endothelim or, in the case of an intracapsular extraction, it could fall into the back of the eye damaging the retina.
This dislocation problem was solved by Jan G. F. Worst with his "iris medallion lens." This iris fixated lens also uses a pair of posterior loops, L-shaped when viewed from the side, which clip behind the iris. The optical portion of the lens is centered in the pupil. A haptic rim surrounds the optical portion and overlies the front of the iris. The haptic rim is of non-uniform width, and has two holes in its widest region. The surgeon uses these holes to suture the lens directly to the iris. This locks the lens in place, and eliminates some of the problem of lens dislocation. The Worst iris medallion lens is the most widely accepted form of intraocular lens. It is estimated that over twenty thousand such lenses have been implanted in humans in the United States alone.
Although the suturing of the iris medallion lens solved the dislocation problem, the use of sutures is itself advantageous. Sutures are subject to biodegradation and/or displacement and are surgically difficult to implement. It is an object of the present invention to provide a lens the placement, fixation and centration of which is accomplished without suturing.
Other attempts have been made to accomplish this objective. The Choyce Mark VIII anterior chamber lens is a thin, generally flat unitary structure having the appearance, when viewed frontally, of an elongated rectangle having rounded corners and notched ends. The rounded corners seat in the angle, and center the plano-convex or biconvex optical portion in front of the pupil. The lens is easy to implant, and thus has gained acceptance by many surgeons. However, cases of CME have been noted with these lenses. Also, tension is placed on the angle, resulting in tenderness to the eye, particularly when rubbed.
Another form of self-centering lens was developed by Barraquer, initially for anterior chamber use and later adapted for placement in the posterior chamber. This lens includes a pair of hook-shaped flexible loops coming off of opposite sides of the optical portion. Since one end of each loop is free, the loops would flex sufficiently to snap in place. When installed in the anterior chamber, the hooks seated in the angle.
Shearing adapted the Barraquer design for use in the posterior chamber. With extracapsular extraction, the hooks may be implaced within the cleft of the capsule. However, during implantation the hooks are held under tension, and when released may fly up behind the iris and seat directly against the ciliary body. Alternatively, with extracapsular extraction, the hooks may intentionally be installed against the ciliar body.
A disadvantage of such an implant is that the hooks continuously exert tension on the ciliary body. An increase in the occurrence of retinal detachments has been noted amongst patients having such Shearing or Barraquer posterior chamber lenses. It is likely that the retinal detachments are associated with the tension exerted on the side of the eye in the vicinity of the ciliary sulcus. Furthermore, tenderness also is noted with such lens when the eye is rubbed.
Thus it is another object of the present invention to overcome these disadvantages by providing a posterior chamber lens in which tension is not exerted on the ciliary sulcus, but which lens nevertheless is self-centering and fixated without suturing.
A further form of posterior chamber lens was developed by Pearce. This lens generally resembles a three-bladed airplane propeller, the blades of which are inserted into the fornix of the capsule after extracapsular extraction. The disadvantage of the lens is that it is of fixed size. Thus the surgeon must take several different sizes into the operating room. If the first does not fit, he must remove this from the eye and insert another of smaller or larger size. It is also recommended to be sutured for centration. The surgical procedure itself is made unnecessarily complex. This disadvantage is overcome by the present invention, another objective of which is to provide a lens implant which will self-adjust to different eye sizes.
Still another form of prior art intraocular lens that is advantageously used with extracapsular extraction is the Binkhorst iridocapsular lens. This is a variant of Binkhorst's iris clip lens, but it does not have anterior loops. The optical portion itself is centered in the pupil with the rim of the lens lightly touching the front of the iris. The single pair of loops, bent slightly rearward, lie behind the lens and are buried in the iridocapsular cleft. After the surgery, the capsule fibroses or develops iridocapsular adhesions which embed part of the posterior loops, thereby giving extra stability to the implanted lens.
Another object of the present invention is to provide a posterior chamber lens that is also stabilized by capsular fibrosis, but in which the lens is situated entirely behind the pupil.
Another problem associated with prior art posterior chamber lenses concerns the difficulty in performing a discission. After extracapsular cataract extraction and intracocular lens implantation, it often becomes necessary to make an opening in the intact posterior capsule. This procedure is difficult when using a posterior chamber lens such as the Shearing, Barraquer or Pearce lens which has a planar rear surface that seats directly against the posterior capsule. To make the discission, the surgeon must sneak in a knife behind the lens to make the cut. This is difficult to do without knocking the lens out of place. A further object of the present invention is to provide a posterior chamber lens which facilitates the performance of a discission.
One of the reasons that a discission of the central posterior capsule may be required is to eliminate clouding due to the growth of lens fibers or of capsular fibrosis posterior capsule subsequent to extracapsular extraction. Although the capsule itself is inanimate, it is virtually impossible to clean off all of the living lens cells when the cataract extraction is performed. As time goes by, the remaining cells continue to grow and proliferate, forming the glistening, bubbly material called Elschnig's pearls. Seeing is impaired, and the discission is required to restore normal sight. In addition to facilitating such discission, a further object of the present invention is to provide a lens which itself is configured to inhibit the growth of lens fibers into the central area of the posterior capsule, so that the lens itself may reduce the incidence of impaired vision due to the growth of fibers or pearls subsequent to lens implantation.
Another potential problem associated with posterior chamber lenses is that their smooth anterior surface might, by anatomical positioning, occlude the pupil. This makes it mandatory that a peripheral iridectomy (i.e., an opening through the iris) be created to provide a fluid flow path from the posterior to the anterior chamber. This iridectomy prevents "pupillary block." It has been found that some iridectomies are incomplete, transitory or unsuccessful. With time, fluid flow may be blocked. A further object of the present invention is to provide a posterior chamber lens which itself allows for the egress of fluid and prevents pupillary block.